High strength thermally resistant ductile cast aluminum alloys

ABSTRACT

The invention relates to a high-strength, thermally-resistant, ductile, cast aluminium alloy (Al Si 7 Mg 0.25 Zr wa, or Al Si 7 Mg 0.25 Hf wa) and (Al Si 6 Mg 0.25 Zr wa or Al Si 6 Mg 0.25 Hf wa), comprising Si: 6.5 to 7.5 wt. % and 5.5 to 6.5 wt.-%, Mg: 0.20 to 0.32 wt. %, Zr: 0.03 to 0.50 wt. % and/or Hf: 0.03 to 1.50 wt. %, Ti: 0 to 0.20 wt. %, Fe: &lt;0.20 wt. %, Mn: &lt;0.50 wt. %, Cu: &lt;0.05 wt. %, Zn: &lt;0.07 wt. % and made up to 100 wt. % with Al. The invention relates to the use thereof for workpieces or parts thereof with elevated thermal loading, such as a cylinder head.

CROSS REFERENCE TO RELATED APPLICATION

This application is a national stage of PCT/EP2004/004654 filed May 3,2004 and based upon DE 103 23 741.0 filed May 24, 2003 under theInternational Convention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a high strength, thermally resistant, tough,ductile cast aluminum alloys containing Zr and/or Hf and their use inthe fabrication of work pieces or parts thereof.

2. Related Art of the Invention

In order to reduce emissions and fuel consumption and to increase thepower output of engines, the combustion pressures and temperatures ofinternal combustion engines, especially for diesel engines, haveincreased over the past few years. These increases lead to moredemanding requirements in terms of the thermo-mechanical load on thework pieces.

According to the state of the art cast aluminum (Al) alloys are known,particularly for the manufacturing of internal combustion engines. CastAl parts from cast Al alloys are utilized widely because of their lowspecific weight, the simple shaping and the ease of manufacturing. Inaddition, it is possible through various casting processes to fabricatecomplicated work pieces or parts thereof such as pistons for internalcombustion engines, cylinder heads, crank cases or engine blocs. Suchwork pieces and particularly certain regions of these work pieces areexposed to high thermo-mechanical loads during operating conditions.

The known high strength and tough cast Al alloy Al Si 7 Mg 0.3 T6, forinstance, reaches its performance limits under these operationalconditions. Because of its Cu content the known cylinder head alloyGK-Al Si 7 Mg 0.3 Cu 0.5 T6 is brittle and susceptible to notch effects.Also of disadvantage is that during tests such cylinder head alloys haveproven to be susceptible to fissuring in the combustion chamber plate(inter cylinder spacing, valve seat, valve channels, glow plug bores)and in the water channels. In addition such Cu containing cast Al alloysform corrosion slurries as a result of their reaction with the coolingfluid used in the cylinder head. In each of the above mentioned cast Alalloys strength enhancing Mg₂Si and Al₂Cu precipitations are formed witha heat treatment, but they are not stable above a temperature of 150° C.and thus are no match for the thermo-mechanical loads of modern engines.Losses in strength of at least 30% occur under long term thermal loadabove 150° C. In addition the precipitation of further Mg₂Si and Al₂Cuphases comes along with an irreversible thermal expansion of thethermally highly stressed regions of the part which reduces the thermalcycling durability of the alloy in operation.

SUMMARY OF THE INVENTION

The objective of the invention is to create a high strength, thermallyresistant and at the same time tough cast Al alloy which at temperaturesequal to or above 150° C. retains its strength values, and whichfeatures a lower thermal expansion through a reduction of phaseformation and thus enhanced thermo-mechanical stability at temperaturesup to 240° C.

For cast Al alloys, usually a grain refining is done with titanium (Ti)which forms Al₃Ti-nuclei within the melt. Since as a consequence of thehigher thermal loads the cast structure of today's alloys for workpieces, in particular in cylinder heads, is susceptible to creeping, thegrain boundaries of the structure need to be stabilized with hightemperature resistant precipitations.

Besides Ti those chemical elements come into consideration which on theone hand promote the growth of a fine grain and on the other handthermally stabilize the fine structure up to temperatures of 250-300° C.through a high temperature stabile grain boundary precipitation and/orsolid solution hardening. It must be pointed out that the fine structureat temperatures above 250° C. is stabilized through high temperaturestable grain boundary precipitations and/or or solid solution hardening.This improves the resistance against creeping and prevents grainboundary sliding and grain growth. Additionally, the desired increasedhigh temperature strength is achieved through solid solution andprecipitation hardening without embrittling the alloy. For a highultimate strain and heat resistance a high content of solid solutionhardening is advantageous.

In order to fulfill the desired requirements according to the inventionintermetallic high temperature phases need to be identified whichparticularly during casting do not form an acicular structure whichembrittles the material.

Starting with the high strength, tough cast Al alloy Al Si 7 Mg T6 thisalloy is, according to the invention, modified through the elements Zrand/or Hf. In a surprising manner Zr and/or Hf fulfill the abovementioned requirements respectively, resulting in connection with Al intemperature stable intermetallic Al₃Zr and Al₃Hf as well as in Zr and Hfcontaining aluminum silicides like Al_(x) Zr_(y) Si_(z), Al_(x)Hf_(y)Si_(z) respectively in Alx (Zr, Hf)_(y) Si_(z) high temperature phaseswith a melting point of 1582° C. (for Al₃Zr) and 1590° C. (for Al₃Hf).Because of their higher temperature stability and very low solubility inAl, Zr and/or Hf, in form of Al₃Zr or, as the case may be, Al₃Hfcontaining Al pre-alloys, are highly efficient as a grain refiningadditive and critically determine the thermo-mechanical strength of thecast Al alloy according to the invention. In a later thermal treatmentadditional high temperature resistant Zr and/or Hf containing aluminumsuicides are formed which thermally stabilize the structure or, as thecase may be, result in a lower irreversible thermal expansion at 240° C.In order to avoid acicular or, as the case may be, brittleprecipitations, Zr or Hf in form of fine Al₃Zr or Al₃Hf phases throughAl pre-alloys with max. 10 wt % Zr and/or Hf needs to be brought intothe Al melt in the form of powder or wire. For grain refining, thesmallest amounts or, as the case may be, a minimum content of 0.03 wt %of Zr and/or Hf is necessary.

Compared to the known Ti grain refining, especially the Zr and/or Hfgrain refining according to the invention are preferred since the Al₃Zrand/or Al₃Hf phases are more temperature stable than Al₃Ti, and inaddition Zr or Hf feature a significantly lower solubility in Al.

With GK-Al Si 7 Mg T6 cylinder heads the effect of grain refining hasbeen demonstrated. Using conventional Ti containing grain refiningadditives like Al Ti 10, Al Ti 5 or Al Ti 3 B 1, a structure with an Aldendrite arm spacing of 20 to 70 μm in the combustion chamber plate hasbeen achieved. Through a Zr or Hf additive of 0.10 to 0.20 wt % in theform of Al₃Zr or Al₃Hf containing Al pre-alloys like Al Zr 5, Al Zr 10,Al Hf 5 or Al Hf 10 in GK-Al Si 7 Mg the Al dendrite arm spacing couldbe economically reduced to 10 to 50 μm. An additional Ti grain refiningleads to a similar small dendrite arm spacing. During the alloying withZr or Hf above 690° C. according to the invention only a fraction of theAl₃Zr or, as the case may be, Al₃Hf phases are dissolved from the Zr or,as the case may be, Hf containing Al pre-alloys such that sufficientAl₃Zr or, as the case may be, Al₃Hf nuclei are left for an Al grainrefining. Depending on the setting rate during the pressure or gravitydie casting the already dissolved Zr and Hf in contrast remainsdissolved in the aluminum. During a later heat treatment consisting ofsolution annealing between 470° C. and 560° C., water quenching and agehardening above 160° C., according to the invention, besides thecomposition of the alloy, also the ratio of solid solution hardening toprecipitation hardening is established. Long solution annealing at hightemperatures with subsequent water quenching results in a high contentof solid solution hardening, whereas long solution annealing attemperatures between 200° C. and 250° C. results in a high precipitationhardening under formation of Zr or Hf containing aluminum silicides likeAl_(x) Zr_(y) Si_(z), Al_(x)Hf_(y) Si_(z) respectively Alx (Zr, Hf)_(y)Si_(z) and reduction of the solid solution hardening. During solutionannealing between 470° C. and 560° C. not more than 0.15 wt % Zr or, asthe case may be, 1.00 wt % Hf can be dissolved. Thus, with respect to anincrease in strength without embrittling the alloy, Hf is preferredcompared to Zr. At engine temperatures (operation temperatures) between150° C. and 250° C. the solid solution hardening remains mostlypreserved. In contrast, under thermal load the precipitation hardeningdecreases through Al₃Zr and/or Al₃Hf. However the reduction in materialstrength is only small respectively smaller compared to the Mg₂Siprecipitation hardening because of the high temperature stability ofAl₃Zr or Al₃Hf from un-dissolved Al grain refining additives and fromlater on formed Zr and Hf containing aluminum silicides like Al_(x)Zr_(y) Si_(z), Al_(x)Hf_(y) Si_(z) or, as the case may be, Alx (Zr,Hf)_(y) Si_(z). With additional Ti grain refining, besides Al₃Zr/Al₃Hfdispersoids Al₃ (Zr, Ti) or Al₃ (Hf, Ti), mixed dispersoids or, inreaction with the silicon, Zr, Hf or Ti containing aluminum silicideslike Al_(x) Zr_(y) Si_(z), Al_(x)Hf_(y) Si_(z) or, as the case may be,Alx (Zr, Hf)_(y) Si_(z) are formed. These precipitations are verytemperature stable so that according to the invention they result in alower irreversible thermal expansion in Zr respectively Hf containing AlSi 7 Mg T6. For instance after 100 h at 240° C. the irreversible thermalexpansion of T7-heat treated cylinder heads made of Zr/Hf-free GK-Al Si7 Mg is between 0.05 and 0.06%. Addition of only 0.10 wt % Zr reducesthe irreversible thermal expansion to approximately 0.04% and with a Zraddition of 0.20 wt % even lower to about 0.025%. According to theinvention the irreversible thermal expansion of T7-heat treated cylinderheads made from GK-Al Si 6 Mg 0.26 Zr/Hf could be reduced an additional10% through a reduced Si content of 4.5 to 6.5 wt %, which is outsidethe range of Al Si 7 Mg (Al Si 7 Mg: 6.5 to 7.5 wt % Si). Furthermorethe ultimate strain of the cylinder head was improved about 1% incomparison to Al Si 7 Mg, so that overall a higher temperature cyclingdurability is achieved. The Si content is preferably in a range between5.5 and 7.5 wt %, particularly preferred between 6.5 and 7.5 wt %.

In order to guarantee the desired high toughness or, as the case may be,resistance against notch effect of the alloy according to the invention,the content of Fe and Mg in the Al Si 7 Mg alloy needs to be restrictedaccording to the invention. According to the invention the followingchemical composition is proposed.

Chemical composition of the cast Al alloy according to the invention(short: Al Si 7 Mg 0.25 Zr T6 or Al Si 7 Mg 0.25 Hf T6 or Al Si 6 Mg0.25 Zr T6 or Al Si 6 Mg 0.25 Hf T6):

-   Si: 4.5 to 7.5 wt %, in particular 6.5 to 7.5 wt %-   Mg: 0.20 to 0.32 wt %-   Zr: 0.03 to 0.50 wt % and/or Hf: 0.03 to 1.50 wt %-   Ti: 0 to 0.20 wt %-   Fe: <0.20 wt %-   Mn: <0.50 wt %-   Cu: <0.05 wt %-   Zn: <0.07 wt %    and in each case brought up to 100 wt % with Al.

As the case may be, residual contaminants (Nb, V, B, Ni, Co) from thefabrication process as known to those working in this art may beincluded.

Additionally Zr, Ti and Hf may also exist as a mixture (e.g. Zr“contaminated” with Hf) in the mentioned areas.

With GK-Al Si 7 Mg 0.25 Zr T6 cylinder heads it has been found thatthrough an addition of Zr of 0.10 to 0.20 wt % the tensile strength aswell as the yield point could be improved by at least 10% withoutembrittling the alloy. That means, the ultimate strain remains unchangedbecause of the Zr grain refining effect. The irreversible thermalexpansion at 240° C. could be reduced by approximately 50%.

The high strength values according to the objective, concurrent withhigh toughness, are furthermore achievable via a fine cast structure.Thus, during casting, high setting rates are desirable. Also, atcomparable cast thicknesses, the structure in gravity die casting andpressure die casting is finer compared to sand casting.

The invention also relates to the use of the above mentioned cast Alalloy for the fabrication of a work piece or a part thereof. Here “partthereof” means an inherent part of the work piece, such as aconstituent, a casing, a coating or such.

Work pieces are particularly but not exclusively pieces such as pistonsfor internal combustion engines, cylinder heads, crank cases, or engineblocks.

For the fabrication of cast Al alloys as well as work pieces accordingto the invention, customary processes as they are known to the personworking in this industry can be utilized unless defined otherwiseherein.

1-5. (canceled)
 6. A cast aluminum alloy comprising Si: 5.5 to 7.5 wt %Mg: 0.20 to 0.32 wt % Zr: 0.03 to 0.50 wt % and Hf: 0.03 to 1.50 wt %Ti: 0 to 0.20 wt % Fe: <0.20 wt % Mn: <0.50 wt % Cu: <0.05 wt % Zn:<0.07 wt % and Al to make 100 wt %, said alloy containing intermetallicAl₃Zr and/or Al₃Hf high temperature phases.
 7. A cast aluminum alloyaccording to claim 6, wherein Zr and Hf exist in a mixture in thementioned regions.
 8. A cast aluminum alloy according to claim 6,wherein the alloy contains Al₃(Zr, Ti) and/or Al₃(Hf, Ti) phases.
 9. Aninternal combustion engine including a component made of a cast aluminumalloy.
 10. An internal combustion engine according to claim 9, whereinsaid component is selected from the group consisting of pistons,cylinder heads, crank cases and engine blocs, and parts thereof.